Variable optical attenuator

ABSTRACT

First and second optical fibers are opposed to each other, between which first and second lenses constituting a lens system having an optical axis coincident with those of the optical fibers are arranged with a gap therebetween in the direction of the optical axis. The actuators, ect are used to move the first and second lenses with electrostatic forces, in opposite directions along the optical axes of the optical fibers by the same amount at the same time. Thereby, the spot size of the light incident on the optical fiber on the reception side is changed while maintaining the light propagating between the first optical fiber and the second optical fiber point-symmetric in mode field shape. This changes the coupling efficiency between the first optical fiber and the second optical fiber, allowing an adjustment in light power.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a variable optical attenuator of light for usein optical communications.

2. Prior Art

Presently, with the wavelength multiplexing transmission systems inoptical communications, there is known such a system as shown in FIG. 1.This system has a plurality of optical amplifiers 31 arranged on atransmission line 30 of wavelength multiplexed light at relayingpositions so that the optical amplifiers 31 amplify the wavelengthmultiplexed transmission light. Such transmission accompanied with theamplification of wavelength multiplexed transmission light by aplurality of optical amplifiers 31 allows long-distance wavelengthmultiplexing transmission.

In the wavelength multiplexing transmission system shown in FIG. 1, afunction of amplifying multi-wavelength light collectively is requiresof each optical amplifier 31. For the wavelength multiplexingtransmission system to improve in transmission quality, it is alsorequired that the plurality of optical amplifiers 31 each performoptical amplification without a great difference in power among theindividual wavelengths of the transmission light amplified. To reducethis differences in power among the wavelengths of the transmissionlight, it has been suggested that a variable optical attenuator havingthe function of uniformizing the multi-wavelength light to a desiredpower collectively be arranged, for example, in each of the opticalamplifiers 31.

Among the requirements for this type of variable optical attenuator are:{circle around (1)} a constant optical attenuation at each wavelength ofthe multi-wavelength light, or equivalently, no variation or smallervariations in optical attenuation with respect to changes in wavelength:{circle around (2)} an attainable optical attenuation of −30 dB orhigher; {circle around (3)} resistance to high optical input power; and{circle around (4)} compact size.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a compact variableoptical attenuator capable of attaining an optical attenuation of, e.g.,−30 dB or higher, reducing the wavelength dependency of the opticalattenuation, suppressing the occurrence of a polarization dependencyloss, and withstanding high optical input power as well.

To achieve the foregoing object, the present invention provides avariable optical attenuator comprising: a first optical part; a secondoptical part opposed to the first optical part with a predetermined gaptherebetween; and optical coupling efficiency adjusting means foradjusting a coupling efficiency between the first optical part and thesecond optical part while maintaining light propagating between thefirst optical part and the second optical part point-symmetric in modefield shape, wherein the optical coupling efficiency adjusting meansadjusts the coupling efficiency between the first optical part and thesecond optical part for an adjustment in light power.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory diagram showing a configuration example of awavelength multiplexing transmission system;

FIG. 2 is a schematic diagram showing the configuration of essentialparts of a conventional variable optical attenuator (No. 1);

FIG. 3A is a schematic diagram showing the configuration of essentialparts of a conventional variable optical attenuator (No. 2);

FIG. 3B is a graph showing the wavelength dependence of the opticalattenuation of FIG. 3A;

FIG. 4A is a schematic diagram showing the configuration of essentialparts of a conventional variable optical attenuator (No. 3);

FIG. 4B is a graph showing the wavelength dependence of the opticalattenuation of FIG. 4B;

FIG. 5 is an explanatory diagram showing a diffraction pattern of lightspreading out in a point-symmetric fashion;

FIG. 6A is a plan view showing the configuration of essential parts of avariable optical attenuator according to an embodiment of the presentinvention;

FIG. 6B is a side view showing the configuration of essential parts of avariable optical attenuator according to an embodiment of the presentinvention;

FIG. 7 is a graph showing the correlation data between changes infiber-end-to-lens distance and variations in optical attenuation of thevariable optical attenuator of FIGS. 6A and 6B;

FIG. 8 is a graph showing the wavelength dependence of the opticalattenuation in the variable optical attenuator of FIGS. 6A and 6B; and

FIG. 9 is a graph showing the polarization dependency losscharacteristic of the optical attenuation in the variable opticalattenuator of FIGS. 6A and 6B.

DETAILED DESCRIPTION

FIG. 2 shows an example of a conventional variable optical attenuator.In this variable optical attenuator, an optical absorption member 15having a glass substrate 11 and an optical absorption film 12 isarranged on the optical path of light propagating between optical parts,or optical fibers 3 and 4. The glass substrate 11 is placed on an XYplane generally orthogonal to the Z-axis with the direction of theoptical axes of the optical fibers 3 and 4 as the Z-axis. The opticalabsorption film 12 is deposited on the top side of the glass substrate11. The optical absorption film 12 has a thickness distribution over theXY plane, and is formed, for example, to gradually increase in thethickness in the direction of the Z-axis as getting closer to the rightside of the diagram in the X direction. The top side of the opticalabsorption film 12 and the bottom side of the glass substrate 11 aregiven anti-reflecting coatings 13 and 14, respectively.

In this conventional variable optical attenuator shown in FIG. 2, whenthe optical absorption member 15 is moved along the X-axis as shown bythe arrow A in the diagram, the optical absorption film 12 varies in thethickness on the optical path between the optical fibers 3 and 4. Thisvariation in the thickness of the optical absorption film 12 causes achange in optical attenuation. Thereby the optical attenuation iscontrolled.

FIG. 3A shows another example of a conventional variable opticalattenuator. In this variable optical attenuator, Faraday rotators 16 arearranged on the optical path. Birefringent wedge plates 17 and permanentmagnets are arranged to sandwich these Faraday rotators 16 therebetweenin the direction of the optical path. In addition, electromagnets 19 arearranged to sandwich one of the Faraday rotators 16 therebetween in adirection orthogonal to the optical path. The reference numeral 20 inthe diagram represents a wave plate.

In this conventional variable optical attenuator shown in FIG. 3A, thedirection of magnetization of the Faraday rotator 16 is changed by thecurrent applied to the electromagnets 19, so as to control the opticalattenuation by means of the Faraday effect. Here, the relationshipbetween the current applied to the electromagnets 19 and the opticalattenuation is shown in the graph of FIG. 3B with the wavelength oflight as a parameter.

FIG. 4A shows still another example of a conventional variable opticalattenuator. This variable optical attenuator is provided with a linearshutter plate 21 to be placed on the optical path of light emitted froman optical fiber 3, and a moving mechanism 22 of this shutter plate 21.

In this conventional variable optical attenuator shown in FIG. 4A, themoving mechanism 22 moves the shutter plate 21 in the X directions inthe diagram to interrupt the optical path of the light emitted from theoptical fiber 3. The optical attenuation is controlled according to theamount of light interrupted by this shutter plate 21. Here, therelationship between the wavelength of the light propagating through theoptical fiber 3 and the optical attenuation is shown in the graph ofFIG. 4B.

Incidentally, the two types of variable optical attenuators shown inFIGS. 2 and 3A have already been put to practical use. The variableoptical attenuator shown in FIG. 4A has been disclosed in IEEE Journalof Selected Topics In Quantum Electronics, Vol. 5, No. 1,January/February 1999, pp. 18-25.

In the variable optical attenuator shown in FIG. 2, however, theprovision of an optical attenuation of −30 dB or higher by using theoptical absorption film 12, given the present technologies, requiresthat the optical absorption film 12 be increased in thickness. Thisinvolves increasing the width Wx of the optical absorption film 12 tothe order of 1 cm. As a result, there has been a problem of difficultminiaturization of the apparatus because such moving means as a motorfor moving the optical absorption film 12 and the rest becomesindispensable. Furthermore, in the variable optical attenuator providedwith the optical absorption film 12, the optical absorption film 12generates heat when the incident light is high in power. Therefore,there has been another problem that incident light having a power abovea certain extent might destroy the optical absorption film 12.

In the variable optical attenuator shown in FIG. 3A, the opticalattenuation has a great dependence on wavelengths as shown by thecharacteristic lines a, b, and c in FIG. 3B. Accordingly, there has beena problem that even if a desired optical attenuation is obtained at somewavelengths, the desired optical attenuation cannot be obtained at,different wavelengths. Here, the characteristic line a shows the opticalattenuations at a wavelength of 1535 nm, the characteristic line b theoptical attenuations at a wavelength of 1549 nm, and the characteristicline c the optical attenuations at a wavelength of 1565 nm. In addition,since not-shown polarizers and analyzers are required aside from theelectromagnets 19, the Faraday rotators 16, and the permanent magnets18, there have been problems of complicated configuration and difficultminiaturization of the apparatus.

Furthermore, the variable optical attenuator shown in FIG. 4A has alsohad the problem that the optical attenuation has a great dependence onwavelengths as shown by the characteristic lines a and b in FIG. 4B.

The characteristic line a shows the wavelength dependence of the opticalattenuation at varying wavelengths with the shutter plate 21 fixed to aposition where an attenuation of 12.2 dB is obtained at a wavelength of1500 nm. Here, a comparison between the maximum and minimum opticalattenuations in the range of wavelengths of 1500-1600 nm finds adifference of approximately 0.8 dB in optical attenuation.

The characteristic line b shows the wavelength dependence of the opticalattenuation at varying wavelengths with the shutter plate 21 fixed to aposition where an attenuation of 13.5 dB is obtained at a wavelength of1500 nm. Here, a comparison between the maximum and minimum opticalattenuations in the range of wavelengths of 1500-1600 nm finds adifference of approximately 1 dB in optical attenuation.

As described above, in the variable optical attenuator shown in FIG. 4A,the wavelength dependence of the optical attenuation becomes moresignificant as the optical attenuation increases. For obtaining anoptical attenuation of −30 dB, the wavelength dependence of the opticalattenuation becomes still greater than on the characteristic line b.Thus, practical application has been difficult unless this problem aboutthe wavelength dependence of the optical attenuation is solved.

Additionally, in this configuration, the mode field of the light emittedfrom the optical fiber 3 is interrupted by the shutter plate 21.Therefore, a point-asymmetric diffraction pattern spreading because ofdiffraction is formed on the end of an optical fiber at the receptionside. This diffraction pattern spreads out beyond the size of theoptical-fiber end, and the light to be received near the rim of theoptical-fiber end varies in reflectance depending on the direction ofpolarization. This has caused another problem of the occurrence of apolarization dependency loss.

The problem about the wavelength dependence of the optical attenuationis ascribable to the wavelength dependence of the mode field diametersuch that the mode field diameter of the light emitted from the opticalfiber 3 increases as the light propagating through the optical fiber 3shifts toward longer wavelengths, and the mode field diameter decreasesas the light shifts reversely toward shorter wavelengths.

It was found that the polarization dependency loss occurring in such avariable optical attenuator as shown in FIG. 4A tended to increase withan increase in the amount of light interrupted by the shutter plate 21.The reason for this seems that as the amount of light interrupted by theshutter 21 increases, the polarization dependency loss becomesrelatively greater because the mode field shape of the light resultingfrom the diffraction spreads out with further asymmetry and the lightpower decreases as well.

No polarization dependency loss will occur, however, when theabove-described diffraction pattern spreads out in a point-symmetricfashion about the optical axis. For example, as shown in FIG. 5, lightTE-polarized at top and bottom ends is TM-polarized at right and leftends, or reversely, light TM-polarized at the tom and bottom ends isTE-polarized at the right and left ends. Eventually, polaritzation-baseddifferences in reflectance will cancel out each other vertically andhorizontally even near the rim of a diffraction pattern if symmetricalportions lie alike vertically and horizontally. Consequently, there willarise no polarization dependency loss.

In view of this, the present inventor provides a configuration forallowing a change in the optical coupling efficiency between first andsecond optical parts with the mode field shape of light maintainedpoint-symmetric. Hereinafter, a variable optical attenuator according toan embodiment of the present invention will be described with referenceto the drawings.

Initially, description will be given of the configuration of an exampleof the variable optical attenuator according to the present embodiment.

As shown in FIGS. 6A and 6B, the variable optical attenuator accordingto the present embodiment has a silicon substrate 1 as a semiconductorsubstrate. A recess 35 is formed in the center of this silicon substrate1. V grooves 28 and 29 are formed on the silicon substrate 1 at bothsides longitudinally across this recess 35. A first optical fiber 3, ora first optical part, is inserted and fixed to the V groove 28. A secondoptical fiber 4, or a second optical part to be opposed to the firstoptical fiber 3 with a predetermined gap therebetween, is inserted andfixed to the V groove 29. The connecting end of the first optical fiber3 and the connecting end of the second optical fiber 4 are opposed toeach other with a gap of, e.g., about 750 μm.

Two comb-like actuators 7 a and 7 b are arranged on the bottom of therecess 35 in between the first optical fiber 3 and the second opticalfiber 4. Hinges 5 and 6 are formed on these two actuators 7 a and 7 b,respectively, by using semiconductor microfabrication technologies. Afirst lens 8 and a second lens 9 are formed on these hinges 5 and 6,respectively, also by using semiconductor microfabrication technologies.Thus, the first and second lenses 8 and 9 are erected upright on thesilicon substrate 1 via the hinges 5 and 6, etc.

These first and second lenses 8 and 9 are arranged with a predeterminedgap therebetween in the direction of the optical axes of the first andsecond optical fibers 3 and 4 (the Z direction) to constitute a lenssystem having an optical axis coincident with those of the first andsecond optical fibers 3 and 4. That is, the optical axes of the firstoptical fiber 3, the first lens 8, the second lens 9, and the secondoptical fiber 4 are put in agreement with one another. Both the firstand second lenses 8 and 9 have a focal length of about 40 μm, forexample.

The comb-like actuators 7 a and 7 b are formed on the recess 35 of thesilicon substrate 1 by micromachining technologies including asacrificial layer deposition process and an etching process which areknown publicly. In addition, springs 23 for balancing the forces of theactuators 7 a and 7 b and voltage applying means (not shown) forapplying a desired voltage to the actuators 7 a and 7 b are provided.Thereby, lens moving means is constituted of the actuators 7 a and 7 b,springs 23, and voltage applying means for moving the first and secondlenses 8 and 9 arranged on the actuators 7 a and 7 b via the hinges 5and 6. The lens moving means is a kind of micro electro mechanicalsystem (MEMS).

Using the lens moving means, the first and second lens 8 and 9 are movedalong the optical axes of the optical fibers 3 and 4, so that thecoupling efficiency between the first optical fiber 3 and the secondoptical fiber 4 is changed for an adjustment in light power.Specifically, a desired voltage is applied to the actuators 7 a and 7 bso that the actuators move back and forth in the Z direction due toelectrostatic forces, moving the first lens 8 and the second lens 9along the Z direction. Here, the first lens 8 and the second lens 9 aremoved in opposite directions by the same distance at the same time.

Incidentally, at initial positions, both the distance between theconnecting end of the first optical fiber 3 and the first lens 8 and thedistance between the connecting end of the second optical fiber 4 andthe second lens 9, or the fiber-end-to-lens distances, are 330 μm, forexample. The voltage applied to the actuators 7 a and 7 b can be variedup to 100 V. Given that this voltage applied is 100 V, thefiber-end-to-lens distances mentioned above decrease to, e.g., 270 μmeach. FIGS. 6A and 6B show situations with different fiber-end-to-lensdistances.

The variable optical attenuator according to the present embodiment isconfigured as described above. When a voltage is applied to theactuators 7 a and 71 to operate the lens moving means, both the distancebetween the connecting end of the first optical fiber 3 and the firstlens 8 and the distance between the connecting end of the second opticalfiber 4 and the second lens 9, i.e., the fiber-end-to-lens distancesvary by the same amount. This changes the spot size of the lightincident on the optical fiber at the reception side while maintainingthe light propagating between the first optical fiber 3 and the secondoptical fiber 4 point-symmetric in mode field shape, with a change inthe coupling efficiency between the first optical fiber 3 and the secondoptical fiber 4.

Consequently, as shown in FIG. 7, the optical attenuation changes withvarying fiber-end-to-lens distances. In this FIG. 7, the characteristiclines a and b show characteristics at wavelengths of 1530 μm and 1580μm, respectively. As is evident from these characteristic lines a and bof FIG. 7, an optical attenuation of −30 dB and higher is attained. Thechanges in optical attenuation due to varying fiber-end-to-lensdistances are almost identical at a wavelength of 1530 nm and at awavelength of 1580 nm. Given an optical attenuation of −30 dB, thedifference in optical attenuation between wavelengths of 1530 nm and1580 nm is as small a value as approximately 0.36 dB or less. That is,the optical attenuation has little wavelength dependence.

For further examination on the wavelength dependence of the opticalattenuation, the optical attenuation was measured for changes withvarying wavelengths at attenuations around 10, 20, and 30 dB. In result,as shown in FIG. 8, little change was found in the optical attenuationfor wavelength variations ranging from 1530 nm to 1580 nm at any of theattenuations around 10 dB (characteristic line a), 20 dB (characteristicline b), and 30 dB (characteristic line c). That is, it was confirmedthat the optical attenuation has little wavelength dependence.

In addition, the variable optical attenuator according to the presentinvention was examined for polarization dependency losses atwavelengths. Specifically, in view of the fact that the variable opticalattenuator shown in FIGS. 6A and 6B had an insertion loss of 0.3 dB,polarization dependency losses were measured for across the range ofwavelengths of 1530 nm and 1580 nm, with optical attenuations fallingbetween the foregoing value of −0.3 dB and a value of −31.2 dB which wasobtained when 100 V was applied to the actuators 7 a and 7 b. Here, thecoupling efficiency between the first optical fiber 3 and the secondoptical fiber 4 could be changed without the diffraction effect due tothe shutter plate 21 since the optical mode field was not interrupted bythe shutter plate 21 as in the conventional variable optical attenuatorshown in FIG. 4A above. In result, as shown in FIG. 9, the polarizationdependency losses in this wavelength range were suppressed toapproximately 0.2 dB or less at any of the attenuations around 10 dB(characteristic line a), 20 dB (characteristic line b), and 30 dB(characteristic line C).

As has been described, according to the variable optical attenuator ofthe present embodiment, the first lens 8 and the second lens 9constituting the lens system having an optical axis coincident withthose of the first and second optical fibers 3 and 4 are moved by thelens moving means including the actuators 7 a and 7 b, in oppositedirections along the optical axis by the same amount at the same time.This makes it possible to change the spot size of the light incident onthe optical fiber on the reception side while maintaining the lightpropagating between the first optical fiber 3 and the second opticalfiber 4 point-symmetric in mode field shape. As a result, it becomespossible to change the coupling efficiency between the first opticalfiber 3 and the second optical fiber 4 for an adjustment in light power.

In this way, the optical attenuation can be freely controlled to attainan optical attenuation of −30 dB or higher it is also possible to reducethe wavelength dependence of the optical attenuation significantly ascompared to the cases of the conventional variable optical attenuatorsshown in FIGS. 3A and 4A above.

Unlike the conventional variable optical attenuator shown in FIG. 4Aabove in which the optical mode field is interrupted by the shutterplate 21, there is no chance of being affected by the diffractionresulting from the shutter plate 21, nor any diffraction other than thatof spreading out as a Gaussian beam. Therefore, the mode field shapepoint-symmetric about the optical axis can be maintained in the lenssystem, with a drastic suppression of the occurrence of polarizationdependency losses.

Since the optical absorption film 12 is not used as in the conventionalvariable optical attenuator shown in FIG. 2 above, it is possible towithstand high optical input power.

The first and second lenses 8 and 9 formed by using semiconductormicrofabrication technologies are erected upright on the siliconsubstrate 1 via the hinges 5 and 6 which are also formed by usingsemiconductor microfabrication technologies. Therefore, it is possibleto support the lens system consisting of the first and second lenses aand 9 with precision and achieve the miniaturization of the apparatus aswell.

The lens system consisting of the first and second lenses 8 and 9 ismoved with electrostatic forces by using the lens moving means includingthe actuators 7 a and 7 b. Therefore, the fiber-end-to-lens distancescan be controlled with accuracy, to attain, by extension, opticalattenuations of favorable control characteristics. In addition, the useof MEMS for the lens moving means eliminates the need for a motor orother moving means as in the conventional variable optical attenuatorshown in FIG. 2 above. This can contribute to further miniaturization ofthe apparatus.

Moreover, the first and second optical fibers 3 and 4 are fixed to the Vgrooves 28 and 29 formed in the silicon substrate 1, respectively. Thus,the optical axes of these first and second optical fibers 3 and 4 can bealigned to each other with facility and with precision.

Consequently, the variable optical attenuator according to the presentembodiment can be installed, for example, in an optical amplifier forwavelength division multiplexing transmission with the function ofuniformizing multi-wavelength light to a desired power collectively,satisfying all the requirements as a variable optical attenuator forsmoothing amplification characteristics.

Incidentally, the present invention is not limited to the embodimentdescribed above, and may be practiced in various forms.

The foregoing embodiment has been configured so that the first andsecond lenses 8 and 9 are moved with electrostatic forces by using thelens moving means including the actuators 7 a and 7 b. Nevertheless,microelectromagnets may be formed by micromachining technologiesincluding a magnetic film deposition process and an etching process, soas to constitute an electromagnetic drive system in which the first andsecond lenses 8 and 9 are moved with electromagnetic forces from themicroelectromagnets. Even in this case, it is possible as in theforegoing embodiment to control the fiber-end-to-lens distancesaccurately and, by extension, control the optical attenuation withprecision, as well as to contribute to further miniaturization of thevariable optical attenuator.

Moreover, the foregoing embodiment has used the lens system consistingof the first and second lenses 8 and 9. Nevertheless, the number oflenses to constitute the lens system is not limited to two, and shall beset as appropriate. For example, a single lens is applicable. Three ormore lenses may be used. The size and shape of the lens(es) are notlimited in particular, and shall be set as appropriate. Besides, themoving distance of the lens(es) such as the first and second lenses 8and 9 is not limited in particular, and shall be set as appropriate. Forexample, when the lens system consists of a single lens, the variableoptical attenuator can be advantageously simplified in configuration.

Furthermore, the foregoing embodiment has used the first and secondoptical fibers 3 and 4 as the first and second optical parts.Nevertheless, either one of the first and second optical parts may be anoptical part other than an optical fiber. Both may be optical partsother than optical fibers.

What is claimed is:
 1. A variable optical attenuator comprising: a firstoptical part; a second optical part opposed to said first optical partwith a predetermined gap therebetween; and optical coupling efficiencyadjusting means for adjusting a coupling efficiency between said firstoptical part and said second optical part while maintaining lightpropagating between said first optical part and said second optical partpoint-symmetric in mode field shape, wherein said optical couplingefficiency adjusting means adjusts the coupling efficiency between saidfirst optical part and said second optical part for an adjustment inlight power.
 2. The variable optical attenuator according to claim 1,wherein said first optical part or said second optical part is anoptical fiber.
 3. The variable optical attenuator according to claim 1,wherein said optical coupling efficiency adjusting means includes: alens system arranged between said first optical part and said secondoptical part so that an optical axis thereof coincides with those ofsaid first and second optical parts; and lens moving means for moving alens constituting said lens system along the direction of the opticalaxes of said first and second optical parts.
 4. The variable opticalattenuator according to claim 3, wherein: said system is composed of afirst lens and a second lens arranged with a predetermined gaptherebetween along the direction of the optical axes of said first andsecond optical parts; and said lens moving means moves said first lensand said second lens in opposite directions by the same amount at thesame time.
 5. The variable optical attenuator according to claim 3,wherein said lens system is composed of a single lens.
 6. The variableoptical attenuator according to claim 3, wherein said lens moving meansmoves said lens constituting said lens system with an electromagneticforce or an electrostatic force.
 7. The variable optical attenuatoraccording to claim 3, wherein: said lens constituting said lens systemis erected upright on a semiconductor substrate via a hinge; and saidlens and said hinge are formed by using semiconductor microfabricationtechnology.
 8. The variable optical attenuator according to claim 7,wherein said first optical part and said second optical part are fixedonto said semiconductor substrate.